1. Field of the Invention
The present invention generally relates to fiber optic sensors and light modulators and, more particularly, to such devices using long period gratings (LPGs) and tuning thereof for sensing or modulation using ionic self-assembled multilayer (ISAM) coatings.
2. Description of the Prior Art
Optical fiber long period gratings (LPGs) are being heavily investigated as key components of a “next-generation” of optical fiber communication systems. Long period gratings are structures formed within an optical fiber, generally by irradiation of the fiber with ultraviolet light using a mask, which produce diffraction of light such that certain wavelengths are strongly attenuated with extremely high selectivity while other wavelengths are substantially less affected. More specifically, long period fiber gratings (LPGs) couple light between copropagating modes of an optical fiber and, due to their compactness, low insertion loss and low back reflection, have been used as spectral shapers and mode converters in optical fiber communication systems. In addition, LPGs that couple the fundamental mode of a single mode fiber (SMF) to one of its cladding modes are ultra-sensitive to the refractive index of the material surrounding the fiber. This high sensitivity and high resolution have led to extensive investigations of LPGs for use as chemical and biological sensors and other index modulating fiber devices. However, reproducible fabrication of such LPGs is non-trivial; often yielding LPGs which attenuate somewhat different wavelengths without perfect predictability of the wavelength which will be most strongly attenuated or the amount of attenuation of a given wavelength. Therefore, there has been great interest in developing techniques by which LPGs can be fine-tuned in a simple manner after basic manufacture.
There is a rapidly growing demand for chemical sensors and bio-sensors having high sensitivity and ease of use. Uses range from environmental monitoring to medical diagnostics to detection of chemical and bio-chemical warfare agents. Thus, the variety of chemical agents to be detected is similarly wide-ranging: including but not limited to DNA hybridization microarrays, drug/protein interactions, protein/protein interaction and the like, possibly extendable to the sensing of the presence of non-biological agents. However, many known sensors, not necessarily using LPGs, rely on adsorption of or interaction with a particular material and may be of greater or lesser specificity than may be desired and/or may require labeling of materials to be detected with another material and, in any case, may not be capable of re-use after a single episode of detection is performed due to irreversible interactions or the presence of captured materials which are not easily removable.
Optical modulation is the key to fiber-optical communication systems. A further important factor is the insertion loss of the modulator mechanism across optical boundaries from and to optical fiber links. For this latter reason, in particular, LPGs have caused substantial interest since their optical activity is achieved “in-line” within a length of optical fiber such that boundaries at which light loss may occur may, in principle, be eliminated. However, modulation in an LPG requires reversible tuning at extremely high frequencies; foreseeably at and above 150 GHz.
Normally, the index sensitivity of LPGs is attributed to the index of the bulk medium surrounding the fiber, and features with sub-wavelength sizes are not expected to modulate the resonance of LPGs. However, it has been demonstrated that switching and sensing devices could be achieved by using adsorption or desorption of a monolayer of water molecules on the surface of a short period relief grating coupler fabricated on planar waveguides. More recently, it has been reported that resonant shifts in LPGs have been observed with films of sub-wavelength thickness, using Langmuir-Blodgett (LB) films. The observed optical response was relatively small with maximal shifts of 10 nm in wavelength with 400 nm of deposited film. Moreover, LB films are not amenable to practical device construction. This is because the LB technique has demanding requirements of expensive special equipment to precisely control the pressure on the liquid surface and is relatively slow. More significantly, films deposited by the LB technique show poor mechanical and thermal stability because the van Der Waals interaction is the primary binding mechanism.
Ionic self-assembled multilayers (ISAM), on the other hand, are formed by a layer-by-layer deposition technique and exhibit enhanced reliability, stability and film quality in comparison to LB films. Through alternately immersing a charged substrate into anionic and cationic polyelectrolyte aqueous solutions, a nano-scale multi-layer thin film is built by consecutive adsorption of polyanions and polycations onto a solid substrate driven by electrostatic forces. The ISAM fabrication method provides a highly controllable means to build precise, nm-thick films on surfaces (indeed, they can be incorporated on any surface with a minimum charge density, such as metals, glass, or silica). Thus, compared with the LB technique, ISAM technique shows more flexibility on choices of substrate or template and thin-film overlay materials for devices.
Extensive bibliographic information concerning research relevant to the background of the present invention as discussed above is provided in the above-incorporated U.S. Provisional Patent Application. Nevertheless, it has not been shown that films can be developed by the ISAM process which allow LPGs to be used for practical chemical and/or biological agent detection, much less to form a sensor which is reusable or a practical optical modulator in the frequency ranges needed for practical fiber-optic communication systems capable of currently required performance.